The positron lifetimes for various vacancy clusters in Si were calculated within the framework of the two-component electron-positron density functional theory. The effect of the trapped positron on the electron density and on the relaxation of the structure was investigated. The calculations showed that, contrary to the usual assumption, the positron-induced forces did not compensate in general for electronic inward forces. Thus, geometry optimization was required in order to determine positron lifetime accurately. For the monovacancy and the divacancy, the results of the calculations were in good agreement with experimental positron lifetimes, suggesting that this approach furnished good estimates for the positron lifetimes of larger vacancy clusters, required for their correct identification via positron annihilation spectroscopy. In one application, the calculations showed that fourfold trivacancies and symmetric fourfold tetravacancies had positron lifetimes which were similar to monovacancies and divacancies, respectively, and could therefore be confused during the interpretation of positron annihilation experiments.

Two-Component Density Functional Theory Calculations of Positron Lifetimes for Small Vacancy Clusters in Silicon. D.V.Makhov, L.J.Lewis: Physical Review B, 2005, 71[20], 205215 (6pp)